1. Field of the Invention
The present invention relates to an optical modulation/multiplexing circuit, and more particularly to an optical modulation/multiplexing circuit of high-bit-rate optical pulses used for ultra-high-speed optical communications.
2. Description of the Related Art
FIG. 4 is a block diagram showing a configuration of a conventional optical modulation/multiplexing circuit. The circuit comprises an optical modulation signal input port 401 (four channels in FIG. 4), an optical clock input port 402, a multiplexed optical signal output port 403, an optical filter 404, a planar lightwave circuit (PLC) substrate 405, a splitter 406, couplers 407, semiconductor optical amplifiers 408, a CW optical source 409, and a combiner 410.
First, optical modulation signals on a plurality of channels to be multiplexed are input through the optical modulation signal input port 401. FIG. 4 shows an example that inputs the four channel signals with the same bit rate. On the other hand, the optical clock signal with the same repetition rate as the bit rate of the modulation signal is input through the optical clock input port 402. The pulse width of the optical clock signal is narrower than the one time slot of the multiplexed signal.
The optical clock signal is split into optical clock signals of the same number of channels of the input optical modulation signals (four channels in this case) by the splitter 406. The optical clock signals propagate through optical waveguides formed on the PLC substrate, and are combined by the couplers 407 with the optical modulation signals propagating through optical waveguides on the PLC substrate. The optical clock signals and optical modulation signals, which are combined by the couplers 407, are input to the semiconductor optical amplifiers 408. In the semiconductor optical amplifiers 408, a four-wave mixing phenomenon, one of nonlinear optical effects, occurs by the incident optical clock signals and optical modulation signals. Thus, the semiconductor optical amplifiers 408 generate modulated optical pulses with a new wavelength and with optical intensities proportional to the optical intensity products between the optical clock signals and the optical modulation signals.
The wavelength λFWM of the newly generated four-wave mixing light is given by the following equation because of the energy conservation.1/λFWM−1/λsig=1/λsig−1/λclkwhere λsig and λclk are wavelengths of the optical signals and optical clock signal, respectively.
The detail of relationship between each wavelength is closed by Govind P. Agrawal, “Nonlinear fiber optics (second edition)”, Academic Press, 1995, ISBN0-12-045142-5, P.404, “Chapter 10, Parametric Process”.
The CW optical source 409 launches bias light into the individual semiconductor optical amplifiers 408 in advance. This makes it possible to suppress the pattern effect in which the modulation efficiency varies depending on the signal pattern, thereby being able to stabilize their outputs.
The generated four-wave mixing light beams propagate through the optical waveguides along with the optical signals and optical clock signals, and are multiplexed by the combiner 410. In other words, the optical clock signal propagates through the waveguides on the PLC substrate from the splitter 406 whereby the optical signal is split to the couplers 407 whereby optical modulation signals are coupled. The generated four-wave mixing beams propagate through the waveguides on the PLC substrate until multiplexed with the optical modulation signals by the combiner 410. The lengths of the waveguides are designed such that the sum of the relative time difference between the channels through which the optical clock signals propagate and the relative time difference between the channels through which the four-wave mixing beams propagate becomes equal to one time slot of the multiplexed signal between adjacent channels. As a result, the optical modulation pulse outputs of the channels after the multiplexing are placed on the time axis at regular intervals.
FIG. 5 is a graph illustrating the output waveforms after the multiplexing on a time axis, that is, a horizontal axis. As referred to FIG. 5, the time-division-multiplexed output can be obtained by extracting only the four-wave mixing beams from the outputs of the PLC substrate by means of the optical filter 404.
However, the conventional integrated multiplexing circuit has the following problems. First, the conventional multiplexing circuit is comprised by the glass waveguides and the semiconductor optical amplifiers which are nonlinear optical devices on the same substrate. The two components, however, differ in diameters of the optical beams propagating through their waveguides. This causes a coupling loss of about 3 dB at each connection point, thereby degrading the signal-to-noise ratio. In addition, since the semiconductor optical amplifiers generate noise called amplified spontaneous emission noise, they also reduce the signal-to-noise ratio.
In addition, it is necessary for the conventional multiplexing circuit to launch the CW light beams to stabilize the generation of the four-wave mixing in the semiconductor optical amplifiers, which presents another problem of complicating the configuration. Furthermore, as clearly seen from the diagram showing the configuration, the optical waveguides corresponding to the individual channels must be connected to the semiconductor optical amplifiers in implementing the circuit. This offers a problem of increasing time and effort for implementation in proportion to the number of channels.
The present invention is implemented to solve the foregoing problems. Therefore an object of the present invention is to fabricate a plurality of nonlinear optical waveguide devices and silica optical waveguides through a small number of process steps. Another object of the present invention is to achieve the simplification of the fabrication process by using the hybrid integration technique; and stabilization of the operation by reducing the connection loss.